Method for manufacturing an article comprising a refractory a dielectric body

ABSTRACT

A refractory dielectric body is heated with a plasma fireball at conditions which do not result in substantial removal of a surface portion of the body, yet which are sufficient to reduce both surface and bulk impurities. Typically, the body is treated with the plasma in the absence of simultaneous deposition of material onto the body. Advantageously, an isothermal, oxygen or oxygen-containing plasma is utilized. The invention is useful for reducing chlorine impurities by at least about 30% to a depth of at least about 10 μm, with accompanying reduction of hydroxyl impurities. The invention thus provides a useful method for reducing the concentration of impurities that contribute to imperfections during the process of drawing fiber from an optical fiber preform, without requiring substantial removal of the surface of the preform.

CROSS-REFERENCE OF THE APPLICATION

This application is a continuation-in-part of U.S. patent applicationSer. No. 08/939142 (our reference Fleming-Pafchek 35-5), entitled"Method for Fabricating an Article Comprising a Refractory DielectricBody," filed Sep. 29, 1997 now U.S. Pat. No. 5,861,047, the disclosureof which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to fabricating an article comprising a refractorydielectric body, more particularly to fabricating glass optical fiberpreforms.

2. Discussion of the Related Art

As the use of optical fiber has increased, the demand for stronger, moredurable fibers with improved optical properties has similarly grown.Loss mechanisms and structural faults in optical fiber typically resultfrom imperfections and impurities existing in the glass preform fromwhich the fiber is drawn, and much effort has gone into finding ways toreduce, remove, or eliminate these imperfections and impurities.Techniques for removing such imperfections and impurities includemechanical milling and chemical etching, neither of which isparticularly desirable. Mechanical milling often introduces mechanicalstresses into a preform that lead to crack formation, and chemicaletching, while typically removing the intended imperfections andimpurities, often introduces chemical by-products as new impurities. Itis also known to use a plasma torch to remove surface impurities andimperfections, as discussed in U.S. Pat. No. 5,000,771, the disclosureof which is herein incorporated by reference. Each of these methods,however, essentially relies on removing a surface portion of the glasspreform, at which the highest concentration of imperfections andimpurities are normally found.

Yet, bulk impurities are also found in glass bodies. (Bulk indicatesthat the impurities are found throughout the glass material, e.g., bothat the surface and into the interior of a body, as opposed to impuritiesthat primarily lie within a few microns of the surface.) Typical bulkimpurities in silica glass include chlorine and hydroxyl. Both types ofimpurities are known to induce detrimental bubbling during themanufacture of and drawing of fiber from silica glass preforms. Chlorineis typically introduced during fabrication of synthetic glasses fromchlorine-containing compounds, e.g., from SiCl₄, and during a subsequentpurification step of such glasses in chlorine gas. It is possible forsubstantial amounts of chlorine to be introduced. For example, 1000 ppmof chlorine atoms in silica glass is not uncommon. Hydroxyl groups areintroduced into silica glass bodies due to the presence of water insol-gel and other fabrication processes, and due to the common use ofoxy-hydrogen torches. Even natural fused quartz glass will containhydroxyl impurities, although such glass typically does not contain highlevels of chlorine.

In some fabrication processes, it would be desirable to leave thesurface of a glass body substantially intact, yet still reduce surfaceimpurities, and advantageously bulk impurities as well. Thus, a methodfor fabricating an article comprising a refractory dielectric body,e.g., an optical fiber preform, is desired in which both surface andbulk impurities are reduced, with substantially no removal of thesurface of the body.

SUMMARY OF THE INVENTION

It has been found to be possible to reduce impurities in a refractorydielectric body, e.g., a silica optical fiber preform, withoutsubstantial removal of a surface portion of the body. Specifically, therefractory dielectric body is treated with a plasma fireball, e.g., asinduced by a plasma torch, at conditions which do not result insubstantial removal of a surface portion of the body, yet which aresufficient to reduce both surface impurities and bulk impurities to adepth of at least about 10 μm, particularly chlorine and hydroxyl.(Substantial removal indicates removal of more than about 0.1 mm fromthe surface of the body. Refractory indicates a ceramic material ofrelatively low thermal conductivity that is capable of withstandingtemperatures of up to about 1600° C. without essential change.Dielectric indicates an electrically insulating material, i.e., amaterial having a resistivity of about 10⁶ ohm-cm or greater.) It ispossible for the body to be solid, e.g., a rod, or to be hollow, e.g., atube (both the interior and exterior of a tube are capable of beingtreated by the plasma fireball). The process is optionally performed inthe absence of simultaneous deposition of material onto the dielectricbody, e.g., in the absence of deposition of a cladding onto a fiber corerod or of deposition of fiber core material into a cladding tube.

The process is able to achieve at least a 30% reduction in chlorineimpurities to a depth of at least 10 μm, compared to the pre-plasmatreated body, with an accompanying decrease in hydroxyl impurities. Itis also possible to achieve a 300% or even a 3000% reduction in chlorineimpurities to a depth of at least 10 μm, by modifying the processparameters, e.g., the temperature to which the dielectric body isheated. It is also possible to select the process parameters such thatless than 0.1 mm of the surface of the refractory dielectric body isremoved, e.g., less than about 0.05 mm, or such that no portion of thesurface is removed. Advantageously, a fireball such as produced by anisothermal plasma torch is utilized. More advantageously, the plasmafireball comprises an oxygen or oxygen-containing plasma. The plasmaadvantageously heats the surface of the dielectric body to a temperatureof about 1800 to about 2300° C. The surface temperature attained dependson a variety of interdependent parameters, including the power suppliedto the plasma, the type of plasma, the speed at which the plasmafireball traverses the body, the speed at which the body is rotated(where cylindrical), the distance from the fireball to the body surface,and the properties of the refractive dielectric body.

The invention is useful for reducing chlorine and hydroxyl impuritiesfrom bodies such as silica optical fiber preforms. The invention thusprovides a method for reducing surface and bulk impurities thatcontribute to imperfections during the process of drawing fiber from anoptical fiber preform, without requiring substantial removal of thesurface of the preform. Production of tougher fiber exhibiting desirableproperties is thereby possible.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows an isothermal plasma torch useful for an embodiment of theprocess of the invention.

FIG. 1B shows an embodiment of the process of the invention in which theinterior of a refractory dielectric tube is treated by a plasmafireball.

FIG. 2 shows an impurity concentration profile indicating the effects ofthe process of the invention.

FIG. 3 shows an impurity concentration profile indicating the effects ofthe process of the invention.

FIG. 4 shows an impurity concentration profile indicating the effects ofthe process of the invention.

FIG. 5 shows an impurity concentration profile indicating the effects ofthe process of the invention.

FIG. 6 shows an impurity concentration profile indicating the effects ofthe process of the invention.

FIG. 7 shows an impurity concentration profile indicating the effects ofthe process of the invention.

DETAILED DESCRIPTION OF THE INVENTION

According to the invention, a refractory dielectric body, e.g., a silicaglass overcladding tube, a silica glass optical fiber preform, or aplanar waveguide, is heated by use of a plasma fireball, e.g., ascreated by a plasma torch, to reduce surface and bulk impurities, to adepth of at least about 10 μm. It is expected that a variety ofimpurities, in addition to chlorine and hydroxyl, will be reduced by theprocess of the invention. It is possible to use the process of theinvention for refractory dielectric bodies of various shapes, both solidand hollow. Optionally, the process is performed in the absence ofsimultaneous deposition of material onto the dielectric body, e.g., inthe absence of deposition of cladding material onto a fiber core rod orof deposition of fiber core material into a cladding tube.

Advantageously, an isothermal plasma torch is used, isothermalindicating that the ion temperature and electron temperature aresubstantially the same in the plasma. The plasma of an isothermal plasmatorch typically contains electrically conducting regions with a centerregion in which the plasma temperature is the highest. A plasma fireballis defined as the region containing the electrically conducting portionof the plasma, into which the plasma-sustaining electromagnetic energyis coupled. Any chlorine-free isothermal plasma capable of substantialultraviolet (UV) emissions is expected to be useful. Examples includeoxygen and oxygen-containing plasma, e.g., oxygen/argon. While notlimiting the invention to any theory, it is believed that reduction ofimpurities is attained by the thermal effects of the plasma incombination with the UV radiation generated by the plasma. It ispossible that the UV radiation affects impurities such as chlorineand/or hydrogen/hydroxyl within the refractory dielectric body, allowingthe impurities to diffuse out of the body.

An example of an isothermal plasma torch 10 useful primarily fortreating solid bodies or the outside surface of hollow bodies is shownin FIG. 1A. The torch 10 contains a fused silica mantle 11 connected toa gas source 17. The gas source 17 delivers the gas used for the plasmadischarge into the mantle 11, and the plasma fireball 12 is excited by aradio frequency (RF) coil 19 and RF generator 14. The fireball 12 istypically contained primarily within the torch, with the center 16 ofthe fireball 12 typically located approximately at the middle of the RFcoil 19. As reflected in FIG. 1A, where the refractory dielectric body20 is cylindrical, e.g., an optical fiber preform, the body is typicallymounted to a lathe 21, which is capable of rotating the body 20 duringtreatment. The torch 10 is typically mounted such that it is capable ofbeing vertically adjusted and tilted to allow a desired arrangement ofthe fireball 12 with respect to the body 20. Torch 10 is typicallycapable of lateral movement in order to traverse the body 20. It is alsopossible for the lathe 21 to provide such lateral movement. In addition,as discussed in U.S. Pat. No. 5,000,771, it is possible to configure atorch such that the fireball is pushed further outside the mantle. It istypically unnecessary to use such a configuration in the inventionbecause pushing the fireball outside the mantle generally results infaster removal of the body being treated, an effect inconsistent withthe advantages of the invention.

To treat the inner surface of a hollow body, e.g., a tube, it is alsopossible to use a torch of the type shown in FIG. 1A. To do so, a smallfireball is generated outside the torch and inserted into the tube.Subsequent to insertion, the fireball is enlarged to contact the innersurfaces of the tube by increasing the gas flow and/or the power, and/orby pushing the fireball further out of the torch (as discussed in U.S.Pat. No. 5,000,771). Advantageously, however, the inside of a hollowbody is treated using an apparatus such as shown in FIG. 1B, in whichthe hollow body itself forms the plasma torch. As shown in FIG. 1B,plasma gas from source 32 is directed into a tube 30, and the plasmafireball is generated inside the tube by RF coils 34 (and an RFgenerator 36) surrounding the tube. Either the tube 30 or the group ofcoils 34 are capable of being moved such that the entire inner surfaceof the tube 30 is treated. In addition, the tube 30 (or similar hollowbodies) are typically rotated during treatment to improve the uniformityof the treatment. (As used herein, plasma torch refers to either a torchseparate from the body being treated, as in FIG. 1A, or an apparatussuch as FIG. 1B, in which a hollow refractory dielectric bodyconstitutes a portion of the torch.)

According to the invention, the refractory dielectric body is heated bythe plasma fireball such that impurities such as chlorine and hydroxylare reduced, yet the surface portion (inner or outer) of the body is notsubstantially removed (i.e., not more than about 0.1 mm is removed fromthe surface of the body). The process is effective in reducing chlorineimpurities at least about 30% to a depth of at least about 10 μm, withan accompanying reduction in hydroxyl impurities, compared to thepre-plasma treated body. It is also possible to reduce the chlorineimpurities at least about 300% or at least about 3000%, to a depth of atleast about 10 μm, depending on the particular process parameters. (Asdiscussed herein, when comparing the impurity level, at a given depth D,of an untreated body to a plasma-treated body, any surface materialremoved by the plasma treatment is not considered. In other words, inthe plasma-treated body, the level of impurities is measured at depth Dfrom the new surface of the treated body--the amount of material removedfrom original surface is not taken into account to modify D.)

An important factor in attaining impurity reduction without substantialsurface removal appears to be the temperature to which the surface ofthe refractory dielectric body is heated by the plasma fireball. Thissurface temperature is affected by a combination of variables, includingthe type of plasma (e.g., higher ionizing gases, which require morepower to free up electrons, have higher energy transfer to a body, andgases also have varying thermal conductivities), the power supplied tothe plasma torch, the translational speed of the fireball with respectto the body, the rotational speed (if any) of the body, and theseparation between the fireball and the surface of the body. (Theseparation between the fireball and surface of the body is defined asthe shortest distance from the center of the fireball to the surface ofthe body.) These variables are inter-related. For example, a slowtranslational speed and large separation will typically have an effectsimilar to a faster translational speed and a smaller separation. Thesurface temperature is also affected by the properties of the refractorydielectric body, e.g., the body's thermal capacity, thermalconductivity, emissivity, and heat of vaporization.

For a silica glass body, the surface of the body is advantageouslyheated to a temperature of about 1800 to about 2300° C. during theplasma treatment, more advantageously about 1900 to about 2100° C.Temperatures lower than 1800° C. generally result in too low a level ofimpurity removal, while temperatures above 2300° C. generally result intoo much surface removal. (As used herein, surface temperature indicatesthe temperature of the body surface at the area just exiting the plasmafireball. The surface temperature is measured, for example, by aiming aninfrared pyrometer at the area of the body just exiting the fireball,where the pyrometer is set at a wavelength of about 4 to about 5 μm andan emissivity of about 0.9 (useful for treatment of outer surfaces of abody), or a brightness pyrometer, similarly aimed at the area justexiting the fireball (useful for inner surfaces of a hollow body).) Thistemperature range is achieved by a combination of the process variablesmentioned above. Specifically, a useful range for plasma torch power(i.e., RF generator power) is about 20 to about 60 kW (typically atabout 3 MHz). Useful traverse rates for the fireball with respect to thebody range from about 0.1 to about 400 cm/min, advantageously about 1 toabout 10 cm/min for an outer surface of a body, and about 50 to about400 cm/min for an inner surface of a hollow body. The traverse rate isattained by moving the torch (for a separate torch, as in FIG. 1A), theRF coils (where the body is hollow and constitutes part of the torch, asin FIG. 1B), and/or the body. A useful range for body rotation (forsolid, cylindrical bodies) is about 20 to about 100 revolutions perminute (rpm). A useful separation distance between the center of thefireball and the surface of the body (primarily for treatment of anouter surface of a body) is about 3 to about 10 cm. (For treatment of aninner surface of a hollow body, however, rotation and distance betweenfireball and surface have a much smaller effect than the traverse rate.)The separation distance will vary depending in part on the overall sizeof the fireball, which in turn varies based largely on the torch power.Specifically, as the power into the fireball lead is increased, a largervolume of gas is ionized at any given instant, thereby increasing thelength and diameter of the fireball. Gas flow into the plasma torch istypically about 1 to about 100 liters/min. A control sample is easilyused to determine the precise parameters for a given set of conditionsthat will yield a desired temperature treatment.

The process of the invention is useful for treating a variety ofrefractory dielectric bodies, including optical fiber preforms, corerods for such preforms, and the inner and/or outer surfaces ofovercladding tubes for such preforms. For example, it is possible totreat the inside of an overcladding tube and the outside of a separatecore rod, prior to collapsing the tube onto the rod to form the finalpreform.

The invention will be further clarified by the following examples, whichare intended to be purely exemplary.

COMPARATIVE EXAMPLE 1

An amorphous silica glass rod, nominally 5 cm in diameter, was made by aprocess that incorporated chlorine and hydroxyl impurities. FIG. 2 showsthe concentration profile for chlorine and hydrogen impurities from thesurface of the rod toward its interior. The profile was measured by SIMS(secondary ion mass spectroscopy). The glass was then treated with anoxy-hydrogen torch. The torch traversed the surface of the rod at about1 to 1.5 cm/min while the rod was rotated at about 45 rpm. The torchheated the surface to greater than 1850° C., as measured by aiming aninfrared pyrometer at the area of the body just exiting the fireball,the pyrometer set at a wavelength of about 4 to about 5 μm and anemissivity of about 0.9. This temperature was sufficient to vaporizesome silica from the surface of the rod. FIG. 3 is the concentrationprofile after treatment with the torch, measured in the same manner.(The surface, i.e., zero, point in FIG. 3 is the new surface of thetorch-treated rod, not the surface point of FIG. 2.) FIG. 3 shows thatthe treatment appears to have slightly reduced the chlorineconcentration, although the reduction is within the margin of error forSIMS. As expected with use of an oxy-hydrogen torch, the hydrogen levelwas not lowered by the treatment.

EXAMPLE 1

The same rod used in Comparative Example 1 was mounted in a glassworking lathe to which a plasma torch was attached. The diameter of therod was measured to an accuracy of 0.001 mm with a laser micrometer. Theplasma torch was stabilized to run an oxygen plasma, and the power tothe plasma was about 40 kW at 3 MHz. The plasma flame was positionedsuch that the fireball would impinge the surface of the glass rod, withthe center of the fireball positioned about 5 to about 6 cm from therod's surface. The surface temperature in the plasma heated region wasmeasured with an infrared pyrometer. The plasma was traversed (over aportion treated by the oxy-hydrogen torch in Comparative Example 1) foran axial length of 50 cm at a traverse rate of 4.0 cm/min while the rodwas rotated at about 45 rpm. The vaporization of the rod was discernedin real time by the extent of re-deposited silica on the rod surfacenear the hot zone, and less surface vaporization occurred than inComparative Example 1. The pyrometer indicated that the plasma heatedthe rod's surface to a temperature of 2020° C., measured in the samemanner as above. After the plasma treatment, the micrometer indicatedthat about 0.02 mm of silica had been removed from the rod surface. TheSIMS concentration profile after the plasma treatment is presented inFIG. 4, which shows that the treatment was effective in reducingchlorine and hydrogen impurities. (The surface, i.e., zero, point inFIG. 4 is the new surface of the plasma-treated rod, not the surfacepoint of FIG. 3.)

EXAMPLE 2

This Example utilized a separate portion of the same oxy-hydrogentreated rod used in Example 1. The oxygen plasma torch of Example 1 wastraversed at a rate of 8 cm/min, with the same power, separation, androtation rate as in Example 1. Two passes were performed at thistraverse rate, duplicating the surface exposure time of Example 1.However, the rod's surface temperature, measured in the same manner asabove, did not exceed 1800° C., and the micrometer indicated that noremoval of silica had occurred. The concentration profile after thetreatment is shown in FIG. 5. While the higher temperature treatment ofExample 1 was more effective in removing impurities, FIG. 5 shows thatsignificant reduction in impurities is possible without removal ofsilica from a rod's surface. (The surface, i.e., zero, point in FIG. 5is the same as prior to treatment, e.g., the same as the surface pointof FIG. 3.)

EXAMPLE 3

The same type and size rod as in the above examples was treated with thesame oxygen plasma torch. The traverse rate, power, separation, androtation rate were the same as in Example 2, except that only a singletraverse was performed, thereby decreasing by about half the exposuretime. The surface temperature of the rod, measured in the same manner asabove, was about 1900° C. The post-treatment laser micrometermeasurements indicated a surface loss of about 0.01 mm. FIG. 6 shows thechlorine and hydrogen concentration profile before the plasma treatment,and FIG. 7 shows the profile after the treatment. (The surface, i.e.,zero, point in FIG. 7 is the new surface of the treated rod.) Thetreatment provided significant reduction of bulk chlorine and hydrogenimpurities.

The trends shown in FIGS. 5, 6, and 7 suggest that the reduction ofchlorine and hydroxyl impurities continues into the silica rod, wellpast the measurement end point of 15 μm.

EXAMPLE 4

A rod of the same type and size as the above examples was treated withthe same oxy-hydrogen torch of Comparative Example 1. Then, the entirerod was placed in an optical fiber draw furnace, and a process ofdrawing 125 μm diameter fiber was begun in which a chlorine- andhydroxyl-containing surface would typically react by forming blisters.Upon visual examination of the rod, blisters were clearly evident. Therod was then removed and further treated with an oxygen plasma torchusing the same traverse rate, power, separation, and rotation rate asExample 3 (with a single traverse). The rod was then placed in the samedraw furnace, and the process of drawing fiber begun. No blisters wereevident upon visual examination.

Other embodiments of the invention will be apparent to those skilled inthe art from consideration of the specification and practice of theinvention disclosed herein.

What is claimed is:
 1. A method for fabricating an article comprising arefractory dielectric body, comprising the steps of:providing arefractory dielectric body; and heating the refractory dielectric bodywith a plasma fireball generated by an isothermal plasma torch withoutsubstantial removal of a surface portion of the body, wherein the gassource for the plasma comprises oxygen, and wherein the heating reducesimpurities in the body, the impurities comprising chlorine and hydroxyl.2. The method of claim 1, wherein the body is solid.
 3. The method ofclaim 2, wherein the body is a rod.
 4. The method of claim 1, whereinthe body is hollow.
 5. The method of claim 4, wherein the body is atube.
 6. The method of claim 1, wherein the body is heated with theplasma fireball in the absence of simultaneous deposition of materialonto the body.
 7. The method of claim 1, wherein the plasma is selectedfrom pure oxygen and a mixture of oxygen and argon.
 8. The method ofclaim 1, wherein the portion of the body heated by the plasma is heatedto a surface temperature of about 1800 to about 2300° C.
 9. The methodof claim 1, wherein the plasma torch has a power of about 20 to about 60kW.
 10. The method of claim 1, wherein during the heating step theseparation between the center of the fireball and the surface of thebody is about 1 to about 10 cm.
 11. The method of claim 2, wherein theheating step is performed at a traverse rate of about 0.1 to about 400cm/min.
 12. The method of claim 4, wherein the traverse rate is about 50to about 400 cm/min.
 13. The method of claim 3, wherein the body iscylindrical and is rotated at a rate of about 20 to about 100 rpm. 14.The method of claim 1, wherein less than 0.05 mm of the surface portionof the body is removed by the heating step.
 15. The method of claim 1,wherein removal of the surface portion of the body is avoided during theheating step.
 16. The method of claim 1, wherein chlorine impurities arereduced at least 30% to a depth of at least about 10 μm.
 17. The methodof claim 16, wherein chlorine impurities are reduced at least 300% to adepth of at least about 10 μm.
 18. The method of claim 17, whereinchlorine impurities are reduced at least 3000% to a depth of at leastabout 10 μm.
 19. The method of claim 1, wherein the refractorydielectric body comprises silica glass.
 20. The method of claim 19,wherein the refractory dielectric body is an optical fiber preform. 21.The method of claim 19, wherein the body is an overcladding tube.
 22. Amethod for fabricating an article comprising a refractory dielectricbody, comprising the steps of:providing a refractory dielectric body;and heating the refractory dielectric body with a plasma fireballwithout substantial removal of a surface portion of the body, whereinchlorine impurities in the body are reduced at least 30% to a depth ofat least about 10 μm.
 23. The method of claim 22, wherein chlorineimpurities are reduced at least 300% to a depth of at least about 10 μm.24. The method of claim 23, wherein chlorine impurities are reduced atleast 3000% to a depth of at least about 10 μm.
 25. The method of claim22, wherein the body is an object selected from a solid object and ahollow object.
 26. The method of claim 22, wherein the body is heatedwith the plasma fireball in the absence of simultaneous deposition ofmaterial onto the body.
 27. The method of claim 22, wherein the plasmafireball is generated by an isothermal plasma torch.
 28. The method ofclaim 27, wherein the gas source for the plasma torch comprises oxygen.29. The method of claim 22, wherein less than 0.05 mm of the surfaceportion of the body is removed by the heating step.
 30. The method ofclaim 29, wherein removal of the surface portion of the body is avoidedduring the heating step.
 31. The method of claim 22, wherein therefractory dielectric body comprises silica glass.